The present invention is directed to a method of and apparatus for the production of processed wood material board or panels, such as chipboard, fiberboard, OSB panels, MDF panels and the like, including coating the wood material particles with a binder resin, such as urea, melamine, or phenolformaldehyde resins. The coated wood material is spread on a support surface in the form of a mat. The mat is transported through a press where the wood material is compacted and hardened due to the presence of a hardener.
Processed wood material board or panels, such as chipboard, fiberboard OSB-panels or MDF-panels comprises wood chips, wood fibers and the like bonded by a binder. Known binders are urea, melamine, and phenolformaldehyde resins. The wood material is scattered to form a mat and is then compressed or compacted in a press to form a compact board or panel. To accelerate the setting or hardening procedure, heat can be supplied during the pressing operation and a hardening agent can be added to the binder before it is applied to the wood material. Increasingly, continuous processes are being used for producing processed wood material panel where in place of a discontinuous multiple platen press, a continuous double band press is used. In such continuous processes, the mat is subjected to an area pressure with the possible addition of heat as the mat is transported through the double-band press so that the processed wood-material panel is produced as a continuous web.
Since wood particles are poor conductors of heat, it has been known in applying binders, which cure or set by the application of heat, to supply hot gases or superheated steam to the mat of wood material in the press for plasticizing the wood particles and for heating the binder. Such a process, which utilizes a continuous band press with two endless revolving press bands, is described in DE-OS 20 58 820. In the process, superheated steam is introduced into the wedge-shaped inlet region of the double band press where the mat is compressed through the medium of pressure plates and the steam-permeable press bands. The pressure plates have nozzles or slots arranged in rows extending transversely of the travel direction of the double-band press for supplying steam and the press bands are formed of a wire mesh or perforated steel strip.
In another Patent Publication DE-OS 34 11 590, an installation is disclosed using a double-band press for fabricating processed wood material panels where the hardening is effected with steam. In this apparatus, distribution and collection channels are arranged on both sides of the press bands in the inlet region of the double-band press with nozzle bores directed toward the pressing gap located between the press bands. Superheated steam is introduced from distributing channels through the press bores into the long sides of the mat. A process utilizing a discontinuous multi-platen press also adds superheated steam through the pressure plates into the mat and is disclosed in Patent Publication DE-OS 34 14 178. To increase the setting speed of the binder and thereby shorten the compressing period, it has been known to add hardening agents or catalyzers to the binder before the binder is applied to the wood material. Strong acids, note U.S. Pat. No. 4,044,087, or bases are known as being particularly violent reacting hardeners for the generally-utilized duroplastic binders.
Because of the increase in the setting speed due to the hardener, an undesirable premature hardening without a corresponding application of pressure may occur with a loose interconnection of the chips while the mat is being formed. To counteract such a situation, usually strong or violent hardeners are not used, rather more slowly reacting hardeners are employed which react only in the presence of large quantities of heat. Such a procedure, however, results in an extended compressing period which leads to a low production output in the discontinuous process. In a continuous production, a long and thus expensive double band press must be utilized because of the long compressing period to achieve somewhat satisfactory production output. The addition of heat by means of steam results in a high humidity content in the processed wood material panel and must subsequently be removed in a drying process. Furthermore, high energy consumption is necessary for the generation of the superheated steam.
Continuous plants for the production of processed wood material panels using steam for increasing the setting speed are known and have certain disadvantages. In the plant disclosed in the Patent Publication DE-OS 34 11 590, the steam is added in the inlet zone through the longitudinal edges of the mat. Since the defusion capacity of the steam across the mat width, that is transversely of the passageway direction between the press bands, is limited, it has been possible to produce only narrow chipboards. The uniform application of steam with a corresponding uniform curing or setting is not assured and results in high rejection rates. Since only narrow processed wood material panels or boards of perfect quality can be produced, the output of such a plant remains small. Accordingly, economical production in such plants is not possible.
In Patent Publication DE-OS 20 58 820, a plant is disclosed providing a more uniform distribution of steam across the width of the apparatus and a more uniform setting or curing across the width of the processed wood material boards, however, the arrangement for supplying steam through the pressure plates and the pressure bands into the mat is very expensive. If the press bands are supported at the pressure plates by rollers or rolls, the rollers rotate in a steam atmosphere, exposing the rollers to a great danger of corrosion. Usually, the lubricant used for the rollers tends to decompose in a steam atmosphere so that after a short operating period, the rollers become scored. In the past, such presses have been found to be undependable in operation and, accordingly, are hardly utilized.